Warning: WOT

If your horse is sweaty and rearing because she doesn't want to enter the trailer, she's not trusting YOU to go where you tell her to go.  Whether it's an "I don't want to go and you can't make me" or an "I'm scared to go so I won't"...it all boils down (I think) to the same thing...you're not the leader. 

I'm hyper sensitive to what kind of control people have of their horses because at one point in time, I had so little control of Holly in the saddle and I was so panicked.  And being around other riders who only "think" they have control just made it worse.  What I see, most of the time, are horses that are trained enough to know that in X situation, they do Y.  They learn to load in a certain type of trailer or handle a certain type of situation...but it's "surface" learning in that it doesn't really translate to a different trailer or a different situation.  Eventually, a horse encounters enough different types of trailers or been despooked with enough objects and situations, that the lack of that initial "detailed" training doesn't matter any more.

Ideally, the type of trailer, obstacle, location, or situation shouldn't matter...the horse should be listening to your cues well enough that go forward (or any other directions) means just that...and having grain, a butt rope, whip, or anything else isn't necessary.

Honestly, I hate the "park the trailer in the field and feed from it" suggestion.  It's NOT about the horse deciding that that particular trailer is ok.  It's about the horse accepting you as the boss/leader...not deciding, on her own, that, ok, she'll go in the trailer for you...because at some point, there'll be something ELSE that she doesn't want to do and you still won't be the one making the decisions...she will.

So in your situation, I'd say to quit thinking about the trailer and start focusing on whether or not she's listening to your cues.  THAT's what needs to be rewarded...not whether or not she'll stand quietly at the trailer or step a foot in.  Her attention needs to be 100% on you.  Because something about the trailer is freaking her, the trailer is going to draw her attention and it shouldn't.  It's like when you're on the trail and YOU see something that you think might spook your horse...so then the horse spooks.  But if YOU aren't concerned about it, the horse isn't as concerned either because she's relying on you to tell her what's spooky and what isn't.  So by "forcing" the trailer issue, in a way, you're telling her that it's A BIG DEAL.

So start 10 ft back (or 20 or 30, if needed).  I'd work in a circle so it's a "go forward" cue vs a "follow me" cue.  Ask your mare to move around you, but be VERY precise.  She needs to go in the direction you asked her to go, not drop her shoulder into your space, not lean on the lead, etc.  Obviously, as she passes between you and the trailer, she's likely to drop toward you because you're "safer"...don't let her.  It's NOT about the trailer...it's about her moving in the direction and speed you want.  She needs to look at you, not have her head away from you (watching the trailer).

Most people, when they lunge/circle a horse, let them have a nose in the air, eyes toward the outside of the circle, generally just moving in a circle but putting almost NONE of their attention on the owner.  wrong.  YOU need to be the center of her world.

Once you get that, start working closer to the trailer.  Again, it's NOT about the trailer.  As soon as she doesn't respond to you, correct her.  Obviously, as she gets closer to the trailer, it's going to be harder, so be sure to reward/praise when she does it right.  But ideally, you should be able to ask her to do anything around that trailer...back toward it, side pass toward it, walk toward it...and her focus is on YOU.  Once that happens, then you can take her TO the trailer...but again, don't make the trailer the reward or the punishment.  If you ask her to take a step toward the trailer and she doesn't, correct her.  She gets to relax and chill when she's DONE WHAT YOU ASK HER TO DO...not just because she's near the trailer.

I know that some people like to use the "safe place" method of training.  As in, if the horse is near or looking at the trailer, they get to relax and be calm.  I understand the philosophy behind that...but to me, that's letting the horse figure out "Oh, if I do this, then I get to stand and do nothing"...which works, because the horse is "figuring out the answer."  But I don't want the horse to have to figure it out because that means she'll have to figure it out the next time it's a situation that's different enough from that one to cause problems.  Instead, I want the horse to say "Oh, she's here, she's in charge, so I can be relaxed no matter what the situation."